Nanoimprint lithography - the past, the present and the future Dongxu Wu1,2, Nitul S Rajput2 and Xichun Luo2,* 1. Centre for Precision Engineering, School of Mechatronics Engineering, Harbin Institute of Technology, Harbin 15001, P. R. China. 2. Centre for Precision Manufacturing, Department of Design, Manufacture and Engineering Management, University of Strathclyde, 75 Montrose Street, Glasgow G1 1XJ, UK. Corresponding author: Prof. Xichun Luo, e-mail: [email protected]; Tel: +44 141 574 5280 Abstract: Nanoimprinting lithography technique uses a very simple concept of transferring pattern of nanoscale features from a mold to a target substrate. In the past two decades, this technique has successfully broken through the barrier of laboratory scale production and become an industrial scale production technique. This article reviews major types of nanoimprint lithography techniques which are mainly base on thermal treatment, ultraviolet light exposure, laser light irradiation or electro chemical reaction. The machining mechanism and components of major nanoimprinting methods are described. A technical comparison of these methods is presented through the analysis of their state-of-the-art research advances. Moreover, the advanced applications of nanoimprint lithography in the fields of optoelectronic devices, solar cells, memory devices, nanoscale sensors and biology are presented. Finally, the article investigates the challenges faced by nanoimprint lithography and its future trend, and discusses the possible solutions to meet these challenges. Keywords: nanofabrication, nanoimprint lithography, throughput, resolution, roll-to-roll 1. Introduction Currently, the research on high throughput, high precision and low cost fabrication approach for nanostructures has attracted a lot of attentions in the field of nanoscience and nanotechnology. Such fabrication technique is of crucial importance for the widespread and 1 commercialisation of nanotechnology as nanostructures are the key building blocks and driving forces for microchips, microelectronic devices, biosensors and high density magnetic storage media [1–5] to reach their ultimate capacities. As such the integration density of the semiconductor devices and circuitry is continuously growing, the dynamic random-access memories (DRAMs) and microprocessors are experiencing a steady growth by 50% and 35% per year [6]. In early 1990’s, scanning probe microscope (SPM) lithography [7, 8] were proposed to fabricate nanostructures. Even with further technical improvements [9], SPM lithography still suffers from low throughput and is therefore only applicable in a laboratory scale. On the other hand traditional high throughput optical lithography approaches such as, X-ray lithography and extreme ultraviolet (EUV) lithography are limited by the requirement of expensive equipment and mask, as well as the attainable resolution [10] due to Abbe diffraction limit. The breakthrough in mass production of nanostructures was made by Prof. Chou who proposed a nanofabrication technique, named as nanoimprint lithography (NIL) in 1995-1996 at NanoStructure Laboratory [11], University of Minnesota, USA. Since then NIL has evolved quickly to become one of the most promising nanopatterning processes and is considered as the future of nanofabrication. Similar to other step and stamp approaches, the principle of NIL is to transfer the patterns of a given sample (such as mold) to a target substrate. This can be achieved by using mechanical contact, or heating, or curing [12], or by electrochemical reaction. NIL is a fast and low cost production process. With the help of modern NIL techniques, pattern of sub-5 nm resolution can be easily achieved [13]. NIL has some key advantages over other existing lithographic processes [14–16] which are summarized as follows: NIL is a cost-effective fabrication technique which is essential for commercial production. 2 NIL can provide high resolution structures. The resolution of a fabricated pattern is limited by the mold only. NIL is a fast process and NIL can be used to fabricate nanopatterns at a large scale in a short time. Due to these advantages manufactures of hard disk drive and semiconductors find NIL as the most convenient production process. NIL has also been widely used in the fabrication of nanostructures for light emitting devices, memory devices, solar cell, flat panel display, and plasmonic devices, etc [17–20]. Because of its high-resolution dense pattern capability, NIL is also considered as the enabling fabrication technique for future flash based memory devices [16]. The conventional nanoimprint techniques are facing many challenges due to some imperfections. Firstly, the throughput of current approaches should be further improved to meet the dynamic market demand for commercial production. Moreover, even if the desired patterns have very small variation, the molds used in physical nanoimprint must be remanufactured by using reactive ion etching (RIE) [9] or electron beam lithography (EBL) [21]. Thirdly, the adhesion between mold and resist is a significant problem, because the mold and patterns may be broken during the procedure of mold release. This review article briefly walks through the history of imprinting process, and then discusses the major techniques developed for NIL and their applications. Finally, it gives a detailed discussion on the challenges and the future aspects of the nanoimprint lithography technique. The article is segregated into the following sections and subsections: History of imprinting process Major NIL techniques developed Thermal NIL Ultraviolet (UV) NIL Laser-assisted direct imprint(LADI) 3 Nanoelectrode lithography Application of NIL Challenges and the future of NIL 2. History of imprinting process Even though the term “nanoimprint lithography” is new, the concept of imprinting to replicate structure is old. Casting is a manufacturing process where a liquid is poured into a mold and subsequently allowed to solidify it. It has been one of the most popular manufacturing processes estimated to be used by the human civilizations since 6000 BC. The process of inserting a mold into a plastic material to replicate structure has been used by the old civilizations. This process is generally termed as embossing process and has been used to replicate and produce coins. It was also used for letterpress. Later the process of hot embossing was added to create structures on polymer materials [22, 23], ultra-thin teflon-like films [24], DNA separation and detection [25] and micro lens array [26]. The realization of creating nanostructures at a large scale using imprinting process came from Prof. Chou’s work in 1995-1996 [10, 27]. He showed that nanostructures of 25 nm resolution could be achieved using imprinting processes. As shown in the schematic picture (Fig.1), a mold with feature size of few tens nm is used to press on a thermoplastic material substrate which is heated up during the molding process. If the temperature of the material increases above its glass transition temperature, the polymer will behave as a viscous material which helps the molding process. With subsequent RIE, the thickness contrast of the imprinted patterns can be transferred through the entire resist region. In the article published by Chou's group, a silicon dioxide mold was used. The patterns, which had a feature size of 25 nm, were created using EBL technique. Polymethyl methacrylate (PMMA) layer of thickness 50-200 nm was prepared on a Si substrate in order to replicate the pattern [10]. The prepared PMMA layer must have the property of desired mold release (should not adhere to the mold after the 4 release of the mold). After the imprinting, RIE was used to etch away the thin resist layers on the compressed portions to achieve a high quality pattern as shown in Fig. 2.Using this approach, patterns of different shapes of nanometre feature size were created (Fig. 3). Press Mold Thermoplastic Substrate Remove Mold RIE Fig.1 Schematic of the imprinting approach implemented by Prof. Chou [27] (a) (b) Fig.2 Scanning electron microscopy image of a (a) PMMA film having 25 nm hole structure (top view), (b) strip pattern formed on a PMMA film using the imprinting process (perspective view) [10] (Reprinted from ref. [10] with permission) Using this flat imprint lithography technique, fabrication of nanostructures at a large-scale has been successfully demonstrated. Prof. Chou's group subsequently has introduced the concept of roller imprint lithography in 1998, a continuous nanoimprinting process [28].Two types of approaches have been discussed for roller imprint lithography in his paper, which are 5 cylindrical mold method and flat mold method. As shown in Fig.3, in the cylindrical mold method, the mold is prepared on the surface of a cylindrical roller. In the second method, a flat mold of Si is placed on the substrate and a smooth roller is rotated over the flat mold. (a) (b) Roller Roller PMMA PMMA Substrate Substrate Fig.3 Schematic picture showing the roller NIL methods as perceived: (a) cylindrical mold method and (b) flat mold method [28] In roller imprinting process, the temperature of the roller is kept above the glass transition temperature of the resist and the temperature of the resist is kept below its glass transition temperature. This ensures that only the contact region between the mold and the resist is suitable for the imprinting process. Using the first generation of roller NIL, resolution of sub- 100 nm was achieved which was shown in Fig.4 [28]. Fig.4 PMMA nanostructure of sub-100 nm resolution created using roller imprint method [28] (Reprinted from ref. [28] with permission) 6 3. Major NIL techniques developed Since the birth of NIL process two decades back, several research groups have contributed in developing several nanoimprinting methods and NIL has grown rapidly within a short time. Depending on the process used for nanoimprinting, major NIL techniques can be classified into four categories, which are discussed below. 3.1 Thermal NIL 3.1.1 Machining mechanism of thermal NIL In order to print a pattern on the target sample using NIL process, the target material must be softened to take the pattern.